Structural basis of noncanonical polyphenol oxidase activity in DLL-II: A lectin from Dolichos lablab.

Lectins known to possess an additional enzymatic function are
called leczymes. Previous studies reported a unique
polyphenol oxidase (PPO) activity in DLL-II—a leczyme from
Dolichos lablab. DLL-II shares a high sequence and structural
homology with DBL—another leczyme from Dolichos biflorus.
Incidentally, DBL possesses lipoxygenase activity, but not the
PPO activity. Legume lectins usually possess two
metal-binding sites A and B. Although these sites are
conserved in both DBL and DLL-II, site A in DLL-II is occupied
by Mn2+ and site B by Ca2+. In contrast, DLL-II binds Cu2+ and
Ca2+ at sites A and B, respectively. Here, investigating the
structural basis of PPO activity in DLL-II, we find that the PPO
activity is only dependent on Cu2+, but not Ca2+; and the lectin
activity requires only Ca2+. Further, our analysis suggests that
an alternative mechanism of PPO reaction may be operative in
DLL-II, which involves a mononuclear Cu2+ metal center; this is
in contrast to the bi-nuclear Cu2+ metal center commonly
observed in all PPOs. Importantly, structural and
computational approaches employed here, we hypothesize
possible PPO binding sites and the corresponding migration
channels for accessing the active site.